1. Field of the Invention
The present invention relates to a method for producing (meth)acrylic acid. More specifically, the present invention relates to a method for producing (meth)acrylic acid using a process of recovering (meth)acrylic acid as an aqueous solution of (meth)acrylic acid from a (meth)acrylic acid-containing gas mixture produced by the catalytic gas phase oxidation of at least one reactant selected from the group consisting of propane, propylene, isobutylene and (meth)acrolein.
2. Description of the Prior Art
Conventionally, (meth)acrylic acid is obtained by the partial oxidation of propane, propylene, isobutylene and/or (meth)acrolein with a heterogeneous oxidation catalyst in the presence of water vapor. In this oxidation method for producing (meth)acrylic acid, by-product impurities, such as water or unreacted propane, propylene, isobutylene and (meth)acrolein, acetic acid, formic acid, formaldehyde, acetaldehyde, maleic acid, propionic acid, furfural and the like, are generated. Gas containing such by-product impurities is generally collected as a (meth)acrylic acid solution via contact with an absorption solvent, and the solvent is separated by distillation, etc. Then, low-boiling point and high-boiling point components are selectively separated.
Methods for absorbing (meth)acrylic acid from (meth)acrylic acid-containing gas by an absorption solvent, which have been known till now, can be broadly divided into methods using water or aqueous solution as a solvent and methods using an organic solvent.
U.S. Pat. No. 3,932,500 discloses a process comprising absorbing acrylic acid from an acrylic acid-containing product gas with a high-boiling, hydrophobic organic solvent, recovering acrylic acid from the absorbed solution and recycling the solvent to an absorption column. In this process, the concentration of acrylic acid at the bottom the absorption column is as low as 6-15% by weight, the amount of water contained in the absorbed solution is about 5% by weight, and the concentration of acrylic acid in the off-gas from the absorption column is about 1%. This loss of acrylic acid (˜1%) at the top of the absorption column is connected directly with process economy and is the heavy burden in view of the fact that acrylic acid is passed to subsequent processes without loss. Also, in processes with larger-scale production, an economic burden for the loss of acrylic acid will be inevitably increased. In order to increase the absorption of acrylic acid, the flow rate of a solvent for absorption needs to be increased. In this case, however, the concentration of acrylic acid in a solution obtained from the bottom of the absorption column will be reduced so that the flow rate of the solvent to be separated from acrylic acid in subsequent processes will be increased, resulting in inefficiency.
Japanese Patent Laid-Open Publication No. Sho 51-25602 discloses a process comprising absorbing an acrylic acid-containing reaction product gas with water and recycling some of nitrogen, oxygen and water discharged from an absorption column to a reactor in order to adjust the gas concentration required for reaction (see FIG. 4). This process has an advantage in that the circulating supply of water required in the reactor is possible since acrylic acid is absorbed with water in the absorption column. Also, the concentration of acrylic acid at bottom of the absorption column is 40-80% by weight, and generally 60-70% by weight. Furthermore, the loss of acrylic acid vented from the absorption column is lower than that of the above-described absorption process using the organic solvent.
Recent technology on the absorption process using water is now led to a preparation process of (meth)acrylic acid for simplifying subsequent processes. This becomes possible by increasing the concentration of (meth)acrylic acid at the bottom of the absorption column. A high concentration of water (i.e., a low concentration of (meth)acrylic acid) in the absorbed solution will cause a shortcoming in that, in an azeotropic distillation process using an organic solvent which is conventionally used in a water separation process, an azeotropic solvent needs to be introduced at the top of the distillation column at a high flow rate. The reason is that for the same production amount, a large amount of the organic solvent needs to be introduced and condensed, and at the bottom of the distillation column, a large amount of heat needs to be fed.